The central objective of this Project is to use miniprotein mimetics together with the structure of the gp120KD4 complex to design antagonists of the interaction of HIV-1 with human cells. T-cell docking and entry by HIV-1, a major route of cell infection in AIDS, is driven by specific recognition of T-cell surface protein CD4 and chemokine coreceptor by HIV envelope protein gp120. The crystallographic structure of CD4 is known, as is its complex with gp120 and the coreceptor surrogate antibody 17b. The structural nature and interrelationship of gp 120 binding sites for CD4 and coreceptor have been revealed. The advancing high-resolution structural understanding of the protein participants in virus-cell recognition, together with the advancing technology of mimetics design, now make it possible to use miniprotein engineering coordinately with biophysical and biologicalanalysis to obtain an advanced mechanistic understanding of the structural basis of the CD4-gp120interaction process. This mechanistic understanding will help guide the design of new antagonists for HIV infection. The specific aims are: (1) Construct novel miniprotein mimetics of CD4 using synthetic and complementary phage randomization methodologies and optimize their affinities for HIV- 1 envelope; (2) Identify key structural elements in miniprotein mimetics of CD4 that control activation of the coreceptor binding site of HIV-1 Env, and form miniprotein variants which favor Env binding with diminished coreceptor site activation; (3) Determine structural and interaction properties of miniprotein mimetics with trimeric vs monomeric forms of HIV- 1 envelopes, including ectodomain and membrane-associated forms, and refine the topological map of key structural elements needed for HIV-1-cell attachment and attachment antagonists. Overall, this project will yield an advanced definition of key structural elements at the interface of the CD4-gp 120 complex that trigger increased HIV- 1 envelope affinity for co-receptor and consequent cell infection; improved understanding of the CD4-viral envelope-coreceptor molecular machine that drives HIV- 1 attachment and infection of host cells; and molecular leads for new antagonist candidates for antiviral therapy. The miniprotein mimetics strategies derived will be useful to determine the mechanism of, and design antagonists for, other protein-protein interactions, such as the gp41 -gp 120 and chemokine receptor.